Plectomerus is a monotypic genus. A single species in its own genus separate from all other genera,* and that classification has been widely accepted since Ortmann & Walker (1922). That work was among the earliest formal attempts to reconcile the current nomenclature of freshwater mussels with the International Code of Zoological Nomenclature.
We are generally skeptical of monotypic genera. All species are distinct from other species, but genera should reflect information about relationships — groups of species. Sometimes monotypic genera are helpful, but sometimes they are not (e.g., Arkansia). What makes P. dombeyanus so distinctive that two classical anatomists like Ortmann and Walker thought it should be set apart from genera like Amblema and Megalonaias?
Ortmann (1912) had regarded the species now classified in Amblema and Megalonaias as well as P. dombeyanus to belong in the genus Crenodonta based on their shared soft-anatomical characters and similar shell morphology. At the time, though, Ortmann had noted that Frierson (a source of Ortmann’s specimens) had reported that some females of P. dombeyanus brooded their larvae in only the outer gills. The other species of Crenodonta brooded in all four gills. In technical terms, some P. dombeyanus are apparently ectobranchus while others are tetragenous, like the other species of Crenodonta.
Frierson (1914) — an early apologist of crazy Frenchmen — argued that Amblema Rafinesque, 1820 had priority over Crenodonta of Schlüter, 1838. And then Utterback (1915) sequestered the species with zig-zag umbo sculpture into a new genus, Megalonaias. This splitting of mussels with plicate (i.e., broadly wrinkled) shells into new genera forced Ortmann & Walker (1922) to choose a genus for P. dombeyanus. And, they decided — given Frierson’s claim of possibly different brooding anatomy — that it would be better to resurrect Plectomerus Conrad, 1853** than to make the classify it one of the other genera.
Fortunately, Ortmann & Walker (1922) actually fell backwards into the correct answer — or at least the “correctest” answer to-date. P. dombeyanus is not closely related to Megalonaias (Tribe Quadrulini). In fact, in some analyses, P. dombeyanus shares a more recent common ancestor with Lampsilis (Tribe Lampsilini) than Amblema (Tribe Amblemini) (Chapman et al., 2008). Plectomerus dombeyanus is a species with unclear phylogenetic affinities, and to park it in another genus would be more confusing.
Plectomerus is monotypic, and that is the way we like it.
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* Frierson (1927) and others regarded Plectomerus as a subgenus of Amblema, but the sentiment is still the same.
** Conrad (1853) clearly intended Plectomerus as a genus equal in scope to the old Crenodonta, and it included the species now classified as Plectomerus, Amblema, and Megalonaias. However, Conrad neglected to assign a type species, and Ortmann & Walker (1922) applied their ICZN-given right to just pick one. The alternative would have been to pull from obscurity another questionably recognizable Rafinesque† genus, Bariosta Rafinesque, 1820.
† “Rafinesque” is used here as an adjective to describe poorly-described, should-have-been-suppressed taxa. We hope the practice catches on. | 
